Thank you all, at least I've got now an idea. I'm new at this, have a few good sites with good names with .com developing, I'll post them when they are ready, one of them has already nearly 6,000 visitors per month, 99% come trough Google and Yahoo, where this sites appear among the first.
As I cannot found certain words with .com, .net I found some are dropping in the next week with .biz, and I didn't know if this domain names are valuable.
Now I have another question. Are .biz sites indexed by the search engines the same way as .com's etc.????